1. Field of the Invention
The present invention is generally in the field of semiconductor fabrication. More specifically, the invention is in the field of fabrication of capacitors in semiconductor dies.
2. Background Art
High performance mixed signal and RF circuits require high density integrated capacitors. Metal-insulator-metal (“MIM”) capacitors can be considered for use in the fabrication of integrated mixed signal and RF circuits on semiconductor dies. Disadvantageously, typical MIM capacitors have low capacitance density and since RF and mixed signal applications require high capacitance values, the die area consumed by typical MIM capacitors is too large, resulting in increased die cost to the manufacturer and the user.
The capacitance value of a typical MIM capacitor at an applied voltage can be represented by the following Equation 1:C(V)=C0(1+aV+bV2)  (Equation 1)where C0 is the capacitance value of the capacitor when the voltage across the capacitor electrodes is zero, V is a voltage across the two electrodes of the capacitor, a is a linear voltage coefficient and b is a quadratic voltage coefficient. As shown in Equation 1, the capacitance value at an applied voltage depends on its “voltage coefficients,” i.e. its linear and quadratic voltage coefficients. Large voltage coefficients cause an undesirable variation in capacitance. In a conventional MIM capacitor, when the dielectric thickness is reduced to increase capacitance density, the MIM capacitor's voltage coefficients disadvantageously increase. Thus, the undesirable increase in voltage coefficients of a conventional MIM capacitor when reducing dielectric thickness, as well as consumption of significant die area by MIM capacitor plates are significant drawbacks in the use of MIM capacitors in mixed signal and RF applications.
Therefore, a need exists for a high density MIM capacitor for use in mixed signal and RF applications where the MIM capacitor's capacitance value is also less dependent on the voltage applied to the capacitor electrodes.